Propellers Flashcards
Define blade face, blade angle, helix angle, helical twist and angle of attack
Blade face - surface of the propeller blade that corresponds to the flat side.
Blade angle - angle between chord line and plane of rotation
Helix angle - angle between the relative airflow and the plane of rotation
Helical twist - twist along the length of the blade due to blade speed being higher at the tip rather than the root. Allows there to be a more constant angle of attack along the length of the blade.
Angle of attack - angle at of which chord of aircraft wing meets relative airflow.
What are the two types of propellers
Fixed pitch and variable pitch
Explain how propeller pitch affects efficiency at different speeds.
Operates at optimum efficiency only under one set of rpm and airspeed conditions
- Engine/propeller combination inefficient for all settings outside the ideal
- Pilot has only the throttle to control both engine power and propeller rpm
Explain the purpose of the constant-speed (variable pitch) propeller.
Constant speed is used to overcome the disadvantaged of the fixed pitch propellers by being able to vary the pitch of the blades.
So the propeller can be operated closed to the optimum angle of attack over a wider range of airspeeds and engine RPM.
Describe CSU usage
E.g. takeoff and climb with high rpm and relatively low airspeed, blade angle can be changed to fine pitch.
For cruise can be changed to coarse
Describe in broad terms the operation of the constant speed unit (CSU) with changes in power setting and airspeed.
If at a given rpm setting, the throttle is opened(MAP increased), engine torque will increase, and so will rpm.
As soon as CSU sense it coarses blade angle of propeller to match engine torque. Opposite occurs if throttle is closed.
If airspeed is reduced, propeller torque will increase (propeller blade having a higher AoA at low airspeed).
as a result rpm will drop and CSU sense it and will fine the pitch of propeller to match propellor torque with engine torque maintaining the selected RPM.
What happens if airspeed is increased without adjusting the throttle?
CSU automatically coarsens pitch
Describe the correct considerations for handling manifold pressure and propeller controls (6)
- Pitch control moved slowly and smoothly
- Full fine on ground
- Run-up checks, 1800 rpm
- Cycle pitch control from full fine – coarse, 2 or 3 times
- only operate with AFM engine range of rpm/manifold settings
- don’t have the manifold pressure above propeller RPM (overboosting) lead to high CHT and detonation
Describe the correct operations for handling manifold pressure and propeller controls
Increasing power – (mixture) Rich,(propeller rpm) Pitch, (increase MAP) Power
•Decrease power – Power, Pitch, Rich
•Take-off – full fine
•Landing – full fine
Describe the correct operations for handling manifold pressure and propeller controls (single controls)
- This relies on a computer to manage the engine, including manifold pressure and RPM of the propeller. The pilot selects the power required and the computer manages the system
- At the military and commuter end of the scale, this may include a full FADEC system
Describe the forces acting on a propeller when:
(a) windmilling;
Act of aircraft going full fine when aircraft speed decreases.
Drag is increased, torque is the same and TR is increased as its the product of these two.
Relative airflow AoA is changed as it goes with the chord line
Describe feathering
Turning blade to the AoA with the oncoming airflow at which no net propeller torque is produced.
At this position, Minimum drag is produced, Propeller held stationary by brake unit and airflow
Describe reverse thrust
When propeller blades are turned through the fine-pitch stop to a blade angle of about minus 20 degrees and power is applied. Reverse thrust is then obtained.
Basically all forces are in reverse to make you slow down.
Explain propeller centrifugal and aerodynamic twisting moments.
CTM - when the rotation of the propeller creates a centrifugal force on both sides of the blade wanting to make it fine pitch
ATM - the tendency of the aircraft propeller to coarsen when the the centre of pressure forward the pitch-changing axis (middle of propeller)
